Dumbledore's Army, also known as the D.A. or Defence Association, was a secret organisation initiated by Harry Potter, Hermione Granger, and Ron Weasley to teach their fellow Hogwarts students proper Defence Against the Dark Arts in 1995. This organisation was made necessary by the refusal of Dolores Umbridge, the subject's professor during the 1995-1996 school year, to teach anything in her classes other than textbook theory. (Credit - Harry Potter Wikia)

The end of the Wizarding War nearly saw the end of DA, for their purpose had shifted towards helping fight Lord Voldemort. However remaining members and students decided to keep it open with it's original intention. As a practical applications group to learn Defense Against the Dark Arts.